The effects of aging upon the structure, mucosal cell turnover and absorptive function of small intestinal epithelial cells will be studied in well nourished Fisher strain rats, 24 to 27 months old and compared with Fisher rats 4-5 months of age. In order to exaggerate potential changes that occur during adaptation of the aging small intestine, studies will be performed also following the hyperegenerative stimulus of intestinal resection and the hyporegenerative noxious stimulus of starvation. Differences in gut dimensions, composition and the concentration of brush border and cytoplasmic enzymes in aging rats will be sought. The histology of the duodenum, jejunum and ileum of both groups will be examined by light microscopy and the villus surface area and epithelial cell number per unit length of intestine determined. In aged and younger rats, changes in mucosal cell differentiation will be related to altered structure and absorptive functions. Mucosal cell turnover of duodenum, jejunum and ileum will be determined following 3H thymidine administration, radioautography and measurement of crypt DNA labelling index, crypt resident time and the rate of movement of labelled epithelial cells on the villus core. Altered differentiation of epithelial cells will be determined from 3H DNA labelling following thymidine injection and determination of the crypt-villus gradient of marker enzymes. Altered absorptive capacity of a long chain triglyceride, triolein and a hydrophobic fat soluble vitamin, calciferol will be sought and the cause determined. Jejunal and ileal glucose and galactose absorption, Km and Vmax will be studied in-vivo and the epithelial uptake rate of a non-metabolizable carbohydrate, 3-0 methyl glucose will be examined in-vitro. These studies will define structural and functional changes which occured in the small intestine of aging Fisher rats and seek to determine whether altered differentiation of small intestinal epithelial cells is responsible for disturbances in structure and function.